Preheat a large skillet over medium-high heat on your stove. Once the skillet is hot, add the avocado oil and coat the bottom of the pan.
Add the onions, and sauté for 20-25 minutes, stirring occasionally.
While the onions are sautéing, combine the remaining ingredients (except the sausage) in a large bowl and mix to combine.
Preheat your oven to 375º.
Heat a well-oiled large cast iron skillet over medium high heat on the stove. Add the sausage to the cast iron skillet and sautee for 5 minutes.
Add the sautéed onions, and stir.
Pour in the egg mixture, and transfer the skillet to the oven.
Bake for 25-30 minutes or until the eggs are set in the middle of the dish.
Top with crumbled feta cheese if desired.